(iOS) Support for DemiBold alias of SemiBold (font-weight 600)

Summary:
For more information on font weight naming see [this post](https://www.quora.com/What-is-the-difference-between-Medium-Demi-and-Semibold-fonts).

DemiBold is fairly common in font naming. For example, iOS React Native lacks support of "AvNext-DemiBold".

Also removed warning about `NSUInteger` <=> `NSInteger` comparison, by making `i` an `NSUInteger`

Before and after screenshots:

![before-fix](https://user-images.githubusercontent.com/177857/30182567-1dfebcb0-93cc-11e7-9b51-78ef6f41c447.png)

![after-fix](https://user-images.githubusercontent.com/177857/30182570-1f90ea94-93cc-11e7-8f68-008ae648ffbe.png)

Add any DemiBold font to an iOS react native project.  Set fontWeight to `'600'` on a `<Text />` component. The font weight should be applied appropriately.
Closes #15856

Differential Revision: D5800928

Pulled By: shergin

fbshipit-source-id: 9095e3e150847f9cb828aa5d080567846441e55d

React/Views/RCTFont.mm

@@ -50,6 +50,7 @@ static RCTFontWeight weightOfFont(UIFont *font)
       @"regular",
       @"medium",
       @"semibold",
+      @"demibold",
       @"bold",
       @"heavy",
       @"black"
@@ -62,6 +63,7 @@ static RCTFontWeight weightOfFont(UIFont *font)
       @(UIFontWeightRegular),
       @(UIFontWeightMedium),
       @(UIFontWeightSemibold),
+      @(UIFontWeightSemibold),
       @(UIFontWeightBold),
       @(UIFontWeightHeavy),
       @(UIFontWeightBlack)